Sub-array beamforming or microbeamforming involves the use of transmit and/or receive arrays of ultrasonic transducers grouped into sub-arrays. U.S. Pat. No. 5,997,479, incorporated by reference herein, describes one application of microbeamforming in which a plurality of transducer elements are grouped into several transmit sub-arrays, and a receive array includes a plurality of transducer elements grouped into several receive sub-arrays. FIG. 2 of the '479 patent also shows several intra-group transmit processors, connected to the transmit sub-arrays, and which generate a transmit acoustic beam directed into a region of interest, and several intra-group receive processors connected to the receive sub-arrays. Each intra-group receive processor is arranged to receive, from the transducer elements of the connected sub-array, transducer signals in response to echoes from the transmit acoustic beam. Each intra-group receive processor includes delay and summing elements which delay and sum the received transducer signals. A receive beamformer includes several processing channels connected to the intra-group receive processors, and each processing channel includes a beamformer delay which synthesizes receive beams from the echos by delaying signals received from the intra-group receive processor, and a beamformer summer which receives and sum signals from the processing channels.
Further, U.S. Pat. No. 6,013,032, incorporated by reference herein, describes another microbeamformer in which each sub-array of the transducer array is connected to a sub-array beamformer with each sub-array beamformer including a sub-array processor and a phase shift network connected to the output of the sub-array processor (see FIG. 2 and the description thereof). A primary beamformer includes a summing unit which sums the outputs of beamformer channels to which the output of the sub-array beamformers is provided, and thereby provides a beamformer signal that represents the received ultrasound energy along a desired scan line.
The term microbeamformer, as generally used hereafter, describes a sub-array beamformer that is integrated within the handle of the transducer in order to facilitate connection to a very large number of piezo-electric sensor elements arranged in a 2D array.
Such a configuration allows for real-time volumetric imaging, when used in combination with a mainframe beamformer and back-end display subsystem. Instead of integrating the electronics of the sub-array beamformer within a handle of a transducer, they may be arranged in the mainframe. The term microbeamforming could also be applied to 1D arrays.
In microbeamforming, control of the shape of the transmit beam is an important aspect for successful implementations of multi-line imaging transducers, in particular, for real-time volume acquisition where high-order multi-line imaging is required to achieve sufficient volume acquisition rates. Control of the shape of the transmit beam is possible because in current ultrasonic transducers, each element in the transducer array is typically connected to control electronics so that each element is individually controllable.
Also, in microbeamforming and other beamforming applications including a transducer array, only a portion of the total number of elements in the transducer array may be operable at any time. This is referred to as controlling the aperture of the transducer array. The aperture of the transducer array refers to the configuration of the transducer elements that are active at any moment. The electronic control of each element in the transducer allows the transmit and receive signals to be shaped and delayed to provide an appropriate signal for the type of imaging being performed.
Referring to FIG. 8, microbeamformers are often constructed with a plurality of microbeamformer patches 100 with each microbeamformer patch (or sub-array) 100 including at least one and most often a plurality of microbeamformer channels 102. Each microbeamformer channel 102 is connected to a transducer 106 and includes a microbeamformer transmitter 104 for driving the transducer 106 and a microbeamformer receiver 108 for receiving signals from the transducer. Preferably, a delay 110 is also provided to delay the received transducer signals and a control circuit 122 is provided to stimulate the transmitter 104. Additional details about the manner in which microbeamformer channels operate can be found in the patents discussed above. Thus, the microbeamformer has individual transmitters for each microbeamformer channel 102 to provide beam steering and focusing control. While additional transmitters are typically provided on the mainframe 112, these mainframe transmitters are not used to drive the transducers 106. Rather, coaxial cables 114 are connected between receivers 116 on the mainframe 112 and the microbeamformer patches 102 so that the coaxial cables 114 are only used for the receive path and not for the transmit path. A single power supply 118 is coupled to all of the microbeamformer patches 100 via a coaxial cable 120.
In a microbeamformer such as shown in FIG. 8, it is known that by controlling the timing and transmit energy supplied to some or all of the microbeamformer channels (commonly referred to as “transmit beamforming”), the ultrasonic interrogation pulse sent into an object being examined can be shaped to provide, for example, high resolution at various depths. Similarly, by electronically altering the receive weights and delays (referred to as “receive beamforming”), the received energy can be used to form high quality images at various depths.
One manner for controlling the transducer elements is known as apodization. Apodization of an ultrasonic transducer aperture is a gradual reduction of the transmit amplitude and/or receive gain from the center of the aperture to the edges of the aperture with a resultant decrease in beam side lobe levels.
In practice, different apodization methods are applied. For example, it is known to use square wave pulsers with power supply voltages that vary across the active aperture and it is also known to apply a per-channel apodization using wave-shaping transmitters. This capability is obtained through additional complexity in either the power management components or the individual transmitters.
When designing microbeamformers for real-time 3D, space is at a premium because the microbeamformer integrated circuits (ICs) must fit in the handle of the transducer. In addition, power dissipation must be limited because of the difficulty in providing cooling for the microbeamformer electronics. As such, the transmitter in the microbeamformer should have as simple and basic a construction as possible and complex modification of the transmitter to provide apodization should be avoided.
The microbeamformer ICs in one prior art system use unipolar pulsers that provide two levels of apodization on a per-element basis—on or off. There are drawbacks to this system most notably, the apodization is limited and often does not provide for adequate beam sidelobe control. It would thus be advantageous to provide new apodization control techniques for transmission from microbeamformers which would allow for adequate beam sidelobe control without significantly complicating the circuitry that must reside within the transducer handle.
To control the acoustic signal generated by the transducers, some prior art ultrasound imaging systems drive the array elements in the transducer with a simple square wave (boxcar) type voltage excitation signal of varying duration and duty cycle. It is known in the art how to create these voltage excitation signals given a fixed or variable mainframe power supply. Often, the voltage or pulse width is changed to try to alter the amplitude of the acoustic signal. Changing the drive voltage changes the total power that can be supplied to drive the transducer whereas changing the pulse width of the driving voltage alters the way the transducer resonates and different acoustic signal amplitudes are possible. For the purposes of apodization across an array, having different drive voltages on every transducer works well. However, for those drivers commanded to output low voltages, the driver circuits themselves dissipate a lot of energy since the output voltage and the system high voltage bus may be very different. For microbeamformers, this inefficiency cannot be tolerated (due to the associated probe heating) so it would be advantageous to provide an efficient driving technique that allows for different output voltage pulses.
To generate a square wave voltage pulse to the transducer, a transmitter needs to source or sink significant amounts of current in order to charge up the capacitance associated with the transducer. Unfortunately, the current through pull-up and pull-down MOSFET devices is directly proportional to their width, so a very large (wide) device is needed to source or sink large currents. Since space is at a premium in microbeamformers, it would be advantageous to develop a pulsing technique that does not require large driver currents so smaller devices may be used.
It is known in the art of transducer design that the current supplied to a transducer is proportional to the velocity of the face of that transducer and hence of the pressure (acoustic amplitude) developed in the medium being transmitted into. In order to change the apodization across the array, it may be useful to exploit this sensitivity of the transducer drive current while maintaining the relatively small size of the microbeamformer.